Fingerprints and handprints are characterized by a series of ridge-like patterns on the surface of the fingers and palms. Because fingerprints are unique to each person, fingerprint recognition technologies are necessary and integral parts of criminal investigations, and have been used for over 140 years to record and identify individuals based solely on their prints. Beyond use for criminal investigations, fingerprints and other biometrical data are the basis for the design of numerous security systems in the private and public sectors for identification and authentication. Security measures at testing facilities, courthouses, stadiums, borders, and airports have become increasingly more common.
Traditional fingerprinting technologies have consisted of rolling each finger on an ink pad and then rolling the inked finger on a paper form, which is subsequently stored in hardcopy. Newer technologies have taken a direct digital scan of the finger as it is placed against or rolled over a flat scanner, thereby eliminating paper while improving the quality of the image, because comparatively little smudging or smearing occurs, and over- or under-inking is eliminated, increasing the reliability of results. More recently, electronic systems utilizing optical methods to scan fingerprints began to replace wet-ink methods. Optical scanning technologies, which have the ability to capture the entire fingerprint area by rotating the finger from one side to the other as the finger is pressed onto the sensor.
Accordingly, businesses and agencies have started to replace the ink-based technology with inkless technologies where the finger is pressed against the surface of the scanner. Deformation of finger tissue during optical scanning is still significant and causes recognition problems, i.e., reduced reliability of recognition when a finger is pressed against an optical scanner. And as previously mentioned any contact can distort the dimensions of the finger, resulting in a statistically decreased confidence of accuracy. This distortion leads to false positives and negatives, lowering the recognition accuracy rate to approximately 92% (NIJ, “Forensic Sciences: Review of Status and Needs”, 173412, (1999)). Furthermore, complex mathematical models to analyze and compare prints are less reliable using questionable ridge pattern data generated from contact methods.
To address the distortion problem resulting from contact fingerprinting, one study attempted to reproduce the pressure gradient across a finger. Using calibrated silicon pressure sensors, the distribution of pressure across a finger was scanned, pixel by pixel, to generate maps of an average pressure distribution during fingerprinting. Although controlled loading of a finger is possible, reproduction of the same distribution of pressure across a given finger during repeated fingerprinting procedures was not observed. The correlation between the stress and strain of a finger under a load was also investigated by comparing images of a finger with no pressure applied and with various pressures applied. The distribution of pressure found on a finger was not only random, but contradicted expected proportional distribution of pressures.
Aside from distortion, existing optical scanning methods are limited in yet another manner. Conventional image correlation techniques are typically performed using Fast Fourier Transforms (FFTs), image shifting, or optical transformation techniques. These techniques require extensive processing of the images in hardware or software and the computational load increases significantly as the image resolution increases, thereby substantially limiting their use.
As a result, various contact-less techniques have been devised. In one, however, contact-less scanning (of a finger) is somewhat problematic as the scan is done by optical collimators around the light source and the optical detector. Spreading of the light coming out through a slit does not provide a sufficiently thin scan. Another describes a contact-less rotational fingerprint imaging system where the optical scanning line is formatted by a cylindrical lens. The inaccuracy of optical slits and lenses in addition to spreading of the optical beam do not allow a sufficiently thin line for adequate resolution. In various other techniques, while contact-less, inadequate scanning (less than 180 degrees), and inadequate resolution for accurate slice alignment both contribute, together or separately, to distortion and decreased accuracy. Further, current technologies have not resolved the “stretching” distortion inherent in projecting a curved surface onto a two-dimensional plane.
Accordingly, there remains an ongoing search in the art for an imaging method and/or system to better utilize the benefits and advantages available through contact-less scanning.